A speech coder for use, for instance, in mobile radio technology includes a linear predictive coder for coding speech signals with the intention of compressing the speech signals and reducing the redundance normally found in human speech. Speech coders which operate with linear predictive coding are known to the art and are found and described and illustrated, for instance, in U.S. Pat. No. 3,624,302, U.S. Pat. No. 3,740,476 and U.S. Pat. No. 4,472,832. This latter patent specification also describes the use of excitation pulses when forming the synthetic speech copy.
The function of the analysis filter in speech coders is to analyze the incoming speech (in the form of speech samples) and determine the filter parameters that shall be transmitted and transferred to the receiver, together with certain so-called rest signals. The excitation pulses to be used can also be transmitted in the manner described in U.S. Pat. No. 4,472,832. Data relating to filter parameters, rest signals and excitation pulse parameters is transmitted in order to be able to transmit on narrower bands than those required to transmit the actual speech signals (modulated).
The filter parameters, which are often called direct form coefficients, are used in the synthesis filter on the receiver side to predict the transmitted speech signal linearly and to form a synthetic speech signal which resembles the original speech signal as far as is possible.
The use of so-called line spectral frequencies (LSFs) for coding the direct form coefficients, i.e. the filter parameters, when coding speech signals linear predictively has earlier been proposed; see for instance "The Computation of Line Spectral Frequencies Using Chebyshev Polynomials", IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP 34, No. 6, December 1986, pages 1419-1425. In this case, the line spectral frequencies are an alternative to the filter parameters with unambiguous correspondence. The primary advantage afforded by coding the direct form coefficients is that the LSFs directly correspond to the formant frequencies from the oral cavity and can thus be quantized advantageously prior to being transmitted and transferred to the receiver.
As described in the aforesaid article, a sum polynomial and a difference polynomial are formed when converting to line spectral frequencies from the direct form coefficients. Subsequent to having constructed these two polynomials, the roots of the polynomials are calculated and thereafter quantized. The number of roots to be localized and calculated vary with the mathematical order of the LPC-analysis. A 10th order LPC-analysis, which is typical, gives five (5) roots with each polynomial.
The normal calculating procedure, which is described in the aforesaid reference, involves localizing the roots by means of iteration, for instance in accordance with the so-called Newton-Rapson method. Subsequent to having calculated the roots, the roots are quantized and the quantized values are transmitted to the receiver side as filter parameters.